Several widely prescribed tricyclic antidepressants are to be compared to determine if their differential anticholinergic activity (ability to block acetylcholine effects) predicts their interference with memory, learning, and other cognitive functions. Comparisons of measures of peripheral anticholinergic activity are planned to identify the most valid and practical indicators of such central effects of the drugs. Normal volunteers are to be selected for single dose and very brief multiple dose drug trials. Knowledge about the effects of these drugs on cognition in normals is essential in order to proceed with the task of disentangling drug-produced (anticholinergic toxicity) from disease-produced cognitive deficits in patients. The long range objective is more valid assessment and increased understanding of cognitive dysfunctions associated with clinical syndromes (such as depression and age-related dementia), with treatments (such as antidepressant drugs and certain medications prescribed for hypertension, cardiovascular diseases, immunological disorders, peptic ulcer, etc.), or with both.